Spectroscopy is the study of the interaction between matter and electromagnetic radiation as a function of the wavelength or frequency of the electromagnetic radiation. As different substances have different electron arrangements, they interact differently with electromagnetic radiation. A particular substance may therefore have a distinctive absorption spectra in that it absorbs electromagnetic radiation at certain well-defined frequencies but does not absorb radiation at other frequencies. Spectroscopy can therefore be used for the identification of particular substances.
It would be desirable to develop spectroscopy so that it is operable over a broad range of optical frequencies to detect substances at very low concentration values such as parts per million or even parts per billion.
A substance is matter which has a specific composition and specific properties. An element is a substance. A compound, for example a molecule, is a substance. Spectroscopy may be used to identify one or more substances in a mixture.